1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for beam characterization and for construction of practical wavefront sensors for beam characterization, metrology, and other applications.
2. Background Art
In many instances where a laser beam is needed, it is important to know something about the laser beam quality. The beam quality affects how the beam will propagate, as well as how tightly it will focus. Unfortunately, beam quality is a somewhat elusive concept. Numerous attempts have been made to define beam quality, stretching back almost to the invention of the laser. In practice, any one of these measures will have some flaw in certain situations, and many different measures are often used. Among these is the M.sup.2 parameter (space-beamwidth product).
The irradiance (or intensity) and phase distribution of a laser beam are sufficient for determining how the beam will propagate or how tightly it can be focused. Most of the beam quality measurements rely on characterizing the beam from only the irradiance distribution, since obtaining this is a comparably straightforward process. However, if both the irradiance and phase distribution could be obtained simultaneously, then all the information would be available from a single measurement.
In general, phase is measured with an interferometer. Interferometers are sensitive instruments that have been extensively developed. They can be used to measure laser beams by using a shearing or filtered Mach-Zehnder arrangement, and can produce the desired irradiance and phase distribution. Unfortunately, these systems rapidly become complex, and are slow, unwieldy, sensitive to alignment, as well as being expensive.
A Shack-Hartmann wavefront sensor is an alternative method for measuring both irradiance and phase. Such sensors have been developed by the military for defense adaptive optics programs over the last 25 years. This sensor is a simple device that is capable of measuring both irradiance and phase distributions in a single frame of data. The advent of micro-optics technology for making arrays of lenses has allowed these sensors to become much more sophisticated in recent years. In addition, advances in charge coupled device (CCD) cameras, computers and automated data acquisition equipment have brought the cost of the required components down considerably. With a Shack-Hartmann wavefront sensor it is relatively straightforward to determine the irradiance and phase of a beam. This allows not only the derivation of various beam quality parameters, but also the numerical propagation of the sampled beam to another location, where various parameters can then be measured.
M.sup.2 has become a commonly used parameter to generally describe near-Gaussian laser beams. It is especially useful in that it allows a prediction of the real beam spot size and average irradiance at any successive plane using simple analytic expressions. This allows system designers the ability to know critical beam parameters at arbitrary planes in the optical system. Unfortunately, measuring M.sup.2 is somewhat difficult. To date, obtaining M.sup.2 has generally required measurements of propagation distributions at multiple locations along the beam path. Although efforts have been made to obtain this parameter in a single measurement, these still suffer from the need to make simultaneous measurements at more than one location. The present invention permits calculation of the parameter using only a single measurement at a single location.
The following references relate to development of the present invention: A. E. Siegman, "New developments in laser resonators", SPIE Vol.1224, Optical Resonators (1990), pp.2-14; H. Weber, "Some historical and technical aspects of beam quality", Opt.QuantElec. 24 (1992), S861-864; M. W. Sasnett, and T. F. Johnston, Jr., "Beam characterization and measurement of propagation attributes", SPIE Vol. 1414, Laser Beam Diagnostics (1991), pp. 21-32; D. Malacara, ed., Optical Shop Testing, John Wiley & Sons, Inc., 1982; D. Kwo, G. Damas, W. Zmek, "A Hartmann-Shack wavefront sensor using a binary optics lenslet array", SPIE Vol.1544, pp. 66-74 (1991); W. H. Southwell, "Wave-front estimation from wavefront slope measurements", JOSA 70 (8), pp.993-1006 (August, 1980); J. A. Ruff and A. E. Siegman, "Single-pulse laser beam quality measurements using a CCD camera system", Appl.Opt., Vol.31, No.24 (Aug. 20, 92) pp. 4907-4908; Gleb Vdovin, LightPipes: beam propagation toolbox, ver.1.1, Electronic Instrumentation Laboratory, Technische Universiteit Delft, Netherlands, 1996; General Laser Analysis and Design (GLAD) code, v. 4.3, Applied Optics Research, Tucson, Ariz., 1994; A. E. Siegman, "Defining the Effective Radius of Curvature for a nonideal Optical Beam", IEEE J. Quant.Elec., Vol.27, No.5 (May 1991), pp.1146-1148; D. R. Neal, T. J. O'Hern, J. R. Torczynski, M. E. Warren and R. Shul, "Wavefront sensors for optical diagnostics in fluid mechanics: application to heated flow, turbulence and droplet evaporation", SPIE Vol. 2005, pp. 194-203 (1993); L. Schmutz, "Adaptive optics: a modern cure for Newton's tremors", Photonics Spectra (April 1993); D. R. Neal, J. D. Mansell, J. K Gruetzner, R. Morgan and M. E. Warren, "Specialized wavefront sensors for adaptive optics", SPIE Vol. 2534, pp. 338-348 (1995); MATLAB for Windows, v. 4.2c.1, The MathWorks, Inc., Natick, Mass., 1994; and J. Goodman, Introduction to Fourier Optics, McGraw-Hill, (New York, 1968).
The present invention is of a wavefront sensor that is capable of obtaining detailed irradiance and phase values from a single measurement. This sensor is based on a microlens array that is built using micro optics technology to provide fine sampling and good resolution. With the sensor, M.sup.2 can be determined. Because the full beam irradiance and phase distribution is known, a complete beam irradiance and phase distribution can be predicted anywhere along the beam. Using this sensor, a laser can be completely characterized and aligned. The user can immediately tell if the beam is single or multimode and can predict the spot size, full irradiance, and phase distribution at any plane in the optical system. The sensor is straightforward to use, simple, robust, and low cost.
In addition to beam characterization, there are a wide variety of applications for wavefront sensors. These include metrology of surfaces, transmissive media, or other objects, measurement of turbulence or inhomogenous media, and static or dynamic measurement of surface or object deformation. These applications benefit in advances to basic sensor technology and can take advantage of many of the features, strengths, and objects of the methods and apparatuses described herein.
A key advantage of the technologies disclosed herein is their inherent stability and robustness. This results from the extremely compact, robust, and rigid sensors that can be constructed at low cost. This is a significant advantage for a host of applications, including beam characterization and those mentioned in the preceding paragraph. In some cases, a robust, compact sensor enables an application otherwise impossible. Furthermore, the techniques needed for constructing such sensors are not readily apparent, with many subtleties being involved in design aspects that would not be apparent even to those highly skilled in the art.